Leiden University’s Centre for Science and Technology Studies (CWTS) provides free access to bibliometric indicators on scientific journals.
A key indicator offered by CWTS Journal Indicators is the SNIP indicator, which stands for source normalized impact per paper. This indicator measures the average citation impact of the publications of a journal. SNIP corrects for differences in citation practices between scientific fields, allowing for more accurate between-field comparisons of citation impact. CWTS Journal Indicators also provides stability intervals that indicate the reliability of the SNIP value of a journal.
SNIP "measures contextual citation impact by weighting citations based on the total number of citations in a subject field. The impact of a single citation is given higher value in subject areas where citations are less likely, and vice versa.
SNIP indicators are based on Elsevier's Scopus bibliographic database and are available for over 20,000 journals.